My personal theory on why Catholics tend to be Democrats and why Protestants tend to be Republican is this: People like their government to resemble their church.
They find it easier to comprehend a form of government whose structure is familiar and comfortable to them.
Thus Protestants prefer government where the authority is concentrated at the lowest level possible and they are inherently distrustful of a powerful central government.
Catholics, on the other hand, like their government to look like their church: bureaucratic, hierarchical, and with power concentrated in an authoritarian manner that is far removed from the local level.
In a similar vein, Protestants/Republicans tend to invest authority in both Scripture and the Constitution while Catholics/Democrats tend to invest authority in their respective bureaucracies.
Just an observation and thesis on my part.
As an afterthought to what I just posted the Muslims are just the same in that they like their church and their government to be dictatorial, authoritarian, autocratic, intolerant, corrupt, dishonest, bloodthirsty, untrustworthy, and etc.
Of course they will SAY otherwise, but their actions speak louder than words.
This is why democracy and Islam are incompatible.
Catholics look to Europe, and government, a Monarch, a government church, something to be over them.